1. Field of the Invention
The present invention relates to a control device for a three-terminal controllable electric valve such as a transistor.
2. Description of the Prior Art
FIG. 9 is a block diagram mentioned in Mitsubishi Semiconductor Data Book--1984 on pages 6-7 indicating a prior art control device for a three-terminal controllable electric valve. Referring to the drawing, reference numeral 21 denotes a power transformer, 22 a rectifier circuit formed of a diode bridge, 23 and 24 smoothing capacitors, 25 a driving circuit, and 26 a transistor as a three-terminal controllable electric valve.
The transformer 21 includes a primary winding connected to a commercial power source (not shown) and a secondary winding connected to the rectifier circuit 22. An output current from the secondary winding is rectified by the rectifier circuit 22 and smoothed by means of the capacitors 23, 24, and then supplied to the driving circuit 25. The driving circuit 25 supplies a signal for driving the transistor 26 to the base electrode of the transistor when an ON signal from outside is applied thereto.
A serious problem involved in the control device of the above-mentioned structure is that a relatively large transformer 21 and rectifier circuit 22 are required for providing the driving circuit 25 with a necessary current for its operation. Further, to ensure a stable operation of the transistor 26 which operates under a high voltage, the transistor 26 must be isolated from the transformer 21 handling high voltages. As another problem, there was a danger of surge voltages generated due to switching of the controllable electric valve 26 to mix into other control devices as noises through the circuit of the mentioned transformer or the like. Although it was proposed to provide a snubber device to cope with the surge generation, this caused another problem, namely that the electric power loss would increase accordingly.